Google does not always include publication dates in snippet.
1) Sitemap:- Many of wordpress seo plugins, automatically ping to Google Webmaster Tools(GWT), so Googlebot fetch the dates from sitemap directly, it will not look out in your blog post dates.
2) Rss Feed:- If you submitted ATOM feed in GWT, then Google use that info to find out fresh content in your blog. If you did not submitted, Then Google can also fetch if you specified application tag into head tag. Mostly All wordpress include the MIME type into the head tag. So Google can also use that info sometimes.
example:-<link rel="alternate" type="application/rss+xml" title="Matt Cutts: Gadgets, Google, and SEO » Feed" href="https://www.mattcutts.com/blog/feed/" />
3) crawable link:- if your some off pages has high value, and you mention some of links on that page, then Googlebot directly index that linked page, and use your publication date(or any timestamp format) as a snippet.
4) Schema :- Google strongly use this format in snippet. Use Article or BlogPosting schema tags, just like you are using the Open graph protocol.
As per my suggestion, you should use schema (Article or BlogPosting) date tags. It is easiest way to control your dates snippet in the search result.